1. Field of the Invention
This invention relates to an ink follower composition for writing instruments, and a process for its production. More particularly, it relates to an ink follower composition for writing instruments which is a highly viscous fluid provided at the rear end of an ink held in an ink container (tubular ink reservoir) and capable of going forward with consumption of the ink, and a process for its production.
2. Related Background Art
As writing instruments having structure that an ink with which the interior of a pipe-like ink container is filled is led out directly to a pen-tip without being passed through any ink feeder or the like, oil-based ink ballpoint pens are conventionally in general use. The ink applied to the oil-based ink ballpoint pen of this type is an ink having a high viscosity of from thousands to tens of thousands of mPa·s. Because of such a high viscosity, a high writing pressure has been required to be applied for writing. In recent years, water-based ink ballpoint pens comprising combination of the structure of such an oil-based ink ballpoint pen with a water-based ink having a medium viscosity (tens to hundreds of mPa·s) have come into wide use.
The water-based ink of this type has a lower viscosity than the oil-based ink, and may require light writing pressure for writing. It, however, has a disadvantage that because of its low viscosity the ink tends to flow out toward the rear end of the ink container. Accordingly, it is usually indispensable to provide at the rear end of the ink in the ink container an ink follower which is a highly viscous fluid.
The ink follower is chiefly required to have two performances conflicting with each other, the performance of preventing the ink from flowing back (ink flow-back preventive performance) and the performance of follow-up moving together with ink with consumption of the ink to ensure smooth writing (ink follow-up performance). Making the ink follower hard in order to improve the ink flow-back preventive performance may make the ink follow-up performance poor, and making the ink follower soft in order to improve the ink follow-up performance may cause the flow-back of ink with ease.
For the purpose of providing ink follower having these required performances, proposals have been made in variety. For example, Japanese Patent Publication No. 6-47318 discloses an ink follower comprised of liquid polybutene as a base oil and having been thickened with fine-particle silica, metal soap, bentonite or the like. Japanese Patent Applications Laid-Open No. 11-165486 and No. 11-170757 also disclose an ink follower comprised of silicone oil as a base oil and having been thickened with fine-particle silica, benzylidene sorbitol, an amino acid derivative or the like.
In the former of these conventional ink follower, the liquid polybutene used as a base oil has a high affinity for ink components such as dyes, surface-active agents, various additives and so forth, and hence part of the ink follower may migrate into the ink or some component of the ink may migrate into the ink follower, so that the ink follower or the ink may change in physical properties to tend to cause problems such as a lowering of ink flow-back preventive performance, a change in properties of ink, and a change in color of ink.
In the case of the latter ink follower comprised of silicone oil as a base oil, its viscosity may greatly lower when a shear force is applied, and it takes a long time until the original viscosity is restored after the shear force has been taken away. Moreover, where the ink follower is stored over a long period of time, its gel strength may gradually lower to make any sufficient ink flow-back preventive performance not achievable, and there has been a possibility of causing difficulties such as leak of ink when ballpoint pens are left with their nibs up (erect) or undergo a shock.